1. Field of the Invention
The present invention relates to a method of producing a permanent magnet, and more particularly to a method of producing a plastic magnet by molding a plastic material containing ferromagnetic powder through injection molding, compression molding, or the like, in an orientating magnetic field.
2. Description of the Prior Art
Conventionally, isotropic permanent magnets produced through sinter-molding have been used as rotors of miniature electric motors. Such permanent magnets produced through sinter-molding, however, have disadvantages that the moment of inertia is large due to the heavy weight thereof, that faulty products may occur due to cracking and/or chipping caused in the magnets during transportation, in the step of assembling a motor, in the step of press-inserting a rotary shaft into a rotor, etc., and that foreign matter due to chipping caused by the magnets causes motor faults. In order to eliminate the foregoing disadvantages, to rationalize the production steps by reducing the number of parts, and to reduce the cost of production, there have been brought to market plastic magnets which are permanent magnets obtained in such a manner that a material consisting of plastic matrix and ferromagnetic powder is molded through injection molding, compression molding, or the like, in an orientating magnetic field to thereby produce an orientated and magnetized molded body of a permanent magnet.
In a stepping motor which is typical of miniature motors, the rotor is multi-polarized in the direction parallel to the rotary shaft thereof to form about twenty four magnetic poles on the outer circumference thereof. The plastic magnets, on the other hand, have surface magnetic flux density such that they can not reach that of isotropic sintered magnets, and therefore, they are used only in extremely limited range of applications, or otherwise, they are subject to polar anisotropic orientation molding to elevate the surface magnetic flux density.
In performing polar anisotropic orientation molding, in orientation magnetic field equipment, there are such disadvantages that the metal mold is complicated in structure and it is impossible to manufacture a number of products at the same time, resulting in extremely low productivity. Further, it is necessary to maintain the temperature of the metal mold above 60.degree. C. during molding, so that the life of an electromagnetic coil used for generating a magnetic field is not stable.
In the case of radial orientation, on the other hand, there is such a disadvantage that when the molded body has such a large ratio of length of molded body (axial direction) to diameter as exceeding 1, the degree of orientation is extremely reduced, and, even if the orientation degree can be kept high, the magnetic flux density is not uniform in the axial direction so that it is impossible to obtain uniform property of magnetic force.